Possible Orientationally Ordered States of Bucky-Cubane

Abstract

For temperatures above $T^*=130$K C$_{60} \cdot$C$_8$H$_8$ forms a cubic crystal consisting of two interpenetrating fcc sublattices, one of freely rotating Buckys C$_{60}$ and the other of orientationally ordered cubane C$_8$H$_8$. The crystal structure below a discontinuous transition is found to be orthorhombic, but the nature of the ordering of the Buckys has not yet been determined. Here possible orderings of the Buckys consistent with the size and symmetry of the orthorhombic unit cell are analyzed. Most likely inversion symmetry is preserved at the transition, in which case the small number of possible orderings are described. If inversion symmetry is removed, the point group can be C$_{2v}$ which supports ferroelectricity or D$_2$ which can be confirmed by proton NMR measurements.